In a continuous ink jet fluid system, the ink used, which includes a carrier fluid, such as water or a solvent, and dye, is continuously recirculated through the system. As it is recirculated, it mixes with air in the return lines and is maintained under vacuum in the ink reservoir. Evaporation of the carrier fluid due to the air-ink interaction increases the dye concentration of the ink which changes various fluid properties such as viscosity and surface tension. Therefore the optimal parameters used to control the ink jet printing process change as the ink concentration varies. As would be obvious to one skilled in the art, affecting ink properties such as viscosity is detrimental, since the energy required to stimulate filaments is determined partially by the viscosity of the fluid.
Proper dye concentration is essential to the stable operation of an ink jet printhead. The measurement of dye concentration is used to determine the amount of replenisher needed to mix with the ink to compensate for the carrier fluid lost due to evaporation. When printing rates are high, the ink usage rate due to printing is much higher than loss rate due to evaporation, therefore the fluid system can be refilled with ink without significantly affecting the ink concentration.
Alternatively, when little or no printing is being done, the system is in an idle condition and the evaporation rate of the carrier fluid is typically higher than the amount of ink removed during printing. In this instance, then, the ink concentration level increases. A replenishment fluid is needed to bring the ink concentration level down to the proper mixture.
Maintaining ink concentration in a continuous ink jet printing system is known in the art. One prior art method for controlling the ink concentration involves counting the print drops. By knowing the number of drops printed and the typical drop size, derived from hole size, one has a measure of the amount of ink printed; the actuations of the float in the tank are used to determine the volume of fluid lost from the tank; the difference between the amount of fluid lost from the tank and the amount printed is assumed to be vehicle lost due to evaporation. Counting printed drops is an open loop control with no feedback parameters. This scheme is sensitive to the accuracy of the estimated drop volume. Small holes size errors or pressure variations can produce drop volumes larger or smaller than assumed by the control system. Ink losses due to spillage or leakage can also not be accounted for. As a result, this scheme can not maintain the ink concentration required for high print quality applications.
Ink concentration has also been monitored by means of the resistivity if the ink, as in patent U.S. Pat. No. 3,761,953 and European Patent No. 0597628A. Resistivity control also is not precise. The resistivity of the ink is not only affected by the ionic content of the dye but also by ions from impurities in the dye stuff and the ink vehicle. Temperature also affects resistivity and calibration curves are necessary to correlate the concentration and resistivity at various temperatures. Furthermore, the resistivity of the ink also changes with time after the ink has been initially installed in the system. As a result, resistivity is less than an ideal indicator of ink concentration.
Thus, these previous methods of concentration control are indirect and do not measure a property of the ink directly linked to concentration. Absorption of light by the ink is directly tied to the concentration of ink. U.S. Pat. Nos. 5,373,366 and 5,241,189 describe a method for monitoring the concentration of the ink by means of the optical absorption. Ink, being pumped to the printhead, passes through a optical cell consisting of closely spaced transparent glass walls. Light from an LED passes through the ink in the cell and is detected by a photodiode. The amount of light detected by the photodiode depends not only on the concentration of the ink but also on the thickness of the optical cell. For a highly absorbing black ink, a very thin optical cell must be used to allow sufficient light to be detected. The small gap through which the fluid must pass in the optical cell can result in unacceptably high pressure drops at the required flow rates.
To reduce the pressure drop, the width of the cell must be fairly large. The resulting large surface area of the optical plates makes the spacing of the optical plates sensitive to changes in ink pressure. As the absorption of the light in the cell varies exponentially with the spacing, the apparent concentration of the ink, as indicated by the optical absorption in the optical cell, varies with ink pressure.
The optical concentration sensor described in U.S. Pat. No. 5,241,189 uses an infrared light emitting diode. Not all dyes and pigment used in inks are absorbing in the IR. As a result, the '189 patent is not applicable for use with all inks. Even with a visible light spectrum, different inks, especially different color inks, have wildly different light absorption characteristics. To provide the desired ink concentration sensitivity for the different inks, one would need to use optical cells whose thickness must be matched to the ink of choice. This is not a practical option.
The optical concentration sensor described in commonly assigned, co-pending U.S. patent application Ser. No. 09/211,035, provides a novel optical cell which overcomes some of the shortcomings of the prior art optical cells. The optical cell walls comprise transparent cylindrical rods between which the fluid being tested passes. These rods can be positioned sufficiently close to each other that the transmission rate can be measured even for highly absorbing inks. The curvature of the glass rods rapidly increases the thickness of the optical cell away from the measurement zone so that the pressure drop produced by flow through the cell can be acceptably low. By varying the gap between the rods from end to end, this concentration sensor can provide small transmission cell thicknesses for testing fluids with high absorption rates and larger cell thicknesses for testing fluids with lower absorption rates.